Linearly extruded products of the type referred to above are usually manufactured in an extrusion line typically comprising a pay-off device, an extrusion machine, a cooling section and a take-up device for the completed product.
In continuous manufacturing processes of this type, to which the present invention relates, it is a requirement to measure the diameter and wall thickness of the extruded products such as tubes or pipes and, in the case of electrical cables, the eccentricity also, that is the off-set position with respect to coaxiality of the metallic core within the insulating coating of the cables.
The need to monitor these measurements on a continuous basis in an extrusion process is, firstly, to ensure specification conformity and, secondly, that the extruded material is being applied as economically as possible in terms of using only that amount of extrusion material absolutely necessary, thus avoiding waste.
In the prior art available at the time the present invention was conceived, these measurements were carried out by optical means using white light or laser light, but these processes are only capable of measuring the overall diameter of the extruded product. By the use of more than one device, it is possible to measure indirectly wall thickness and eccentricity. Ultra-sonic methods have also been used to measure wall thickness using water as a contact medium.
The use of radioactive beta or x-rays enables the measurement of the wall thickness of an extruded product without contact with it. These methods, however, require special handling by reason of the fact that they involve inherent health hazards as will be readily appreciated.
The invention may also be used in the industrial field of manufacturing flat products, such as, rubber or plastic sheets, insulating tapes, films, paper and the like, thereby to measure the thickness of the material and the overall width of the product being manufactured.
Prior art available in measuring flat products, includes indirect contact methods, whereby two wheels or rollers are placed above and below the product, and the difference of the readings shown by the two wheels, indicates product thickness.
A non-contact optical method has also been used, in which, two “distance measuring devices” are mounted above and below the product. The difference between the two distance readings indicates product thickness.
Both these methods suffer from inaccuracies, which include mechanical wear and wheel bounce in the case of the mechanical contact type and defocussing on the optical type, either on product vibration or product thickness change.
A further limitation of the “contact” and “optical” methods is that they measure, only the thickness along a narrow part of the product width and not the complete area of the flat product sheet.
Alternative measuring methods such as, ultrasonic, radioactive, beta or x-rays are not recommended, since they require special handling and therefore present an inherent health hazard as will be appreciated.
The present invention makes use of terahertz radiation (hereinafter referred to as THz radiation) to irradiate the product as it passes through the rays on its path of travel and to utilize in a time related manner the radiation after passing through the product to determine its dimensional profile.
The frequencies of THz radiation are located between infra-red and micro-waves and the wavelengths of THz radiation are in the range between 30 micrometers and 3 millimeters.
Terahertz radiation (THz) has the advantage in that it behaves in a manner similar to that of white light, that is to say that the radiation can be reflected by mirrored surfaces but is able to penetrate and pass through dielectric or insulating materials such as rubber, paper and various plastics including polyethylene and the like.
The speed of transmission of THz radiation through the dielectric or insulating material is dependent on the chemical composition and material density of the product and this property and a penetrative ability of the THz radiation through dielectric or insulated materials will be used to obtain the measurements required.
The system disclosed herein utilizes an optical system to produce a curtain of THz radiation through which the product passes in a linear fashion in its path of travel.
The transient time or speed of each successive ray in the curtain of rays is used to compute, by matrix imaging methods, the dimensional parameters of the product in particular to determine the thickness of the extrusion coating so as to ensure that the coating thickness meets operational requirements.
Uniformity of the transient times or speeds of the rays through the extruded coating are important in achieving high accuracy of the results of the measuring process.
Due to the optical components of the system in that some rays in the curtain will be travelling at different speeds than others, these transient times or speeds will differ so that the accuracy of the results obtained will vary in dependence on the position of the products in the curtain of rays which occurs due to the swaying motion of the product in its path of travel.